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Slight vent temp rise when moving

terryk on Mon July 21, 2008 6:38 PM User is offline

Year: 86
Make: Pontiac
Model: Fiero
Engine Size: 2.8L
Refrigerant Type: R12
Ambient Temp: 90F
Pressure Low: 26-48
Pressure High: 180-250
Country of Origin: United States

This is more of a curiosity than anything else.

I notice when I am moving along at say 55MPH+plus I see a 3F vent temp increase vs. stopped or when I clutch the engine and let the revs drop.

For example, cruising along I see 47-48 temps but if I slow down or let the engine rev's drop I will se 42-44. If I slow way down or stop at and idle it can get as low as 40F.

Not complaining, more curious about what is happening. The cycle point is about 25 PSI. The high side pressure runs around 180 moving and 200-220 if I stop. If it's hotter where the system doesn't cycle (the pressures stabilize) the vent temps will go down to the 40-44 range. Should I lower the trip point on the cycling switch?

Any thoughts on this phenomenomenomenom-om?


Edited: Mon July 21, 2008 at 6:39 PM by terryk

bearing01 on Wed July 23, 2008 1:31 AM User is offline

bit of a head scratcher for me. I would expect the system to operate cooler on the highway. But you still appear to be working well at those temperatures.

At those vent temp's do you have the blower set to the same speed for the idle or highway scenario? Slower moving air over the evaporator will be cooler. If you got more ram air at highway speeds then maybe that's why it's a little warmer.

Is the clutch cycling at idle? Can you tell if the clutch is cyling at highway speeds? If the compressor is pumping down the evaporator to pressures at/below the cutout pressure maybe the cycling temperatures/pressures to get the compressor to come back on provide average vent temperatures that are higher.

Maybe the faster pully speeds at high rpm are causing the compressor clutch to slip? That would cause the compressor the inability to push as much refrigerant through the system. I don't know the exact spec for your clutch gap but typically it's around 0.020"

terryk on Wed July 23, 2008 10:46 AM User is offline

Same here. I would think the temps would drop. It's a subtle increase. The pressures drops when the car moves so there is air flow. You can feel when it cycles (if it's going to cycle) if you pay attention.

It will cycle at idle if the ambient temps are low enough but it's been over 90F here and the cycling gets real slow. No belt slip that I can tell.

It's not enough of a rise to really get freaked out about but I thought it was interesting. When it's this hot the low side tends to hover in the 26-28 range to it's not going to cycle with the switch set at 25.

Oh well! Still can't complain. Right now I am hoping the oil is circulating.

TRB on Wed July 23, 2008 11:44 AM User is offlineView users profile

Usually this is do to hot air flowing into the vehicle. Check that a grommet or something of that nature is not causing this issue.


When considering your next auto A/C purchase, please consider the site that supports you:

terryk on Wed July 23, 2008 5:09 PM User is offline

I threw my trusty LED on my cycling switch and all things being equal, the additional airflow over the condenser from moving lowers the high side pressure and raises the vent temps 2-3 degrees. I have seals around the condenser and radiator such that any air going through condenser can only go through the radiator so the fan is doing it's work at low speeds but above about 35 the forward motion is most of the airflow......unless I am stalling the air somehow.....hmmmm

I am measuring inside the vent a couple of inches so any holes into the passenger compartment shouldn't matter. Also, the change occurs pretty quick (10-20 seconds or so). I am not recirculating the air (not using GM's infamous MAX)

It doesn't appear to be the cycling switch doing it. Cruising and with the high ambient temps (95F) the system rarely kicked off due to the cycling switch.

Edited: Wed July 23, 2008 at 5:21 PM by terryk

JJM on Thu July 24, 2008 12:27 AM User is offline

* Slight undercharge
* Dirty evaporator (likey, on an '86 especially if it was a smoker vehicle)
* Incorrect (slightly smaller) OT

CPS setting is correct. Do not adjust to lower PSI.


When considering your next auto A/C purchase, please consider the site that supports you:

terryk on Thu July 24, 2008 10:24 AM User is offline

* Slight undercharge - Maybe but it would have to be very slight.

* Dirty evaporator (likey, on an '86 especially if it was a smoker vehicle) - It's been removed and cleaned including the box itself.

* Incorrect (slightly smaller) OT - Factory OT.

I will leave it. The charge is so close to the factory spec that messing with it will probably bring more issues than it's worth.

All done. Later everyone!

Edited: Thu July 24, 2008 at 10:25 AM by terryk

Dougflas on Thu July 24, 2008 11:12 AM User is offline

Monitor the alternator output. Possible that at idle the 13.8 volts is dropping and blower spped also drops.

terryk on Thu July 24, 2008 5:40 PM User is offline

Thanks for the suggestion Doug, I'm running a CS130 and it's pretty solid. There is no sign the dash blower is changing speed.

But since this is the slow time of the AC season I was wondering: When AMA recharges a system do you just dump in the specified amount of refrigerant or do you bother tweaking it up or down? I assume you check that it meets some specification but would you real bother getting it dead on since "dead-on" varies with the ambient conditions?

This plays to the post about when are you really done.

From what I understand, if the system spec is X number of ounces, the factory will dump X in, check that it's cooling to some level and call it good. Who would ever bother checking the vent temps and how they move around? It's cold, it's working, leave it alone.

In my case I used 3 14 oz cans on a 40oz system so the worse it could be over is 2oz. With the hose purges between the cans I find it hard to believe I lost much more than 2oz (bringing me down to 40oz) and on the scales I came in right at 40oz. Maybe I did lose more but it has to be dang close.

The only way to know if I am really over or under is to recover and measure. For a 2oz difference (5%) would anybody really change the charge considering the good vent temps?

Seems to me, no....

Edited: Thu July 24, 2008 at 5:45 PM by terryk

Dougflas on Thu July 24, 2008 8:29 PM User is offline

Could you be losing vacuum while driving under acceleration? Possibly a blend door messing you up?

TRB on Thu July 24, 2008 9:03 PM User is offlineView users profile

For the most part we charge to spec. If you have some odd cooling at that point. We then will adjust it a little. This is very rare when using the refrigerant designed for the vehicle.


When considering your next auto A/C purchase, please consider the site that supports you:

terryk on Thu July 24, 2008 10:14 PM User is offline

Ok, thread closed!

She's working good.

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